Mammalian sperm are not able to fertilize eggs immediately after ejaculation. They acquire fertilization capacity after residing in the female tract for a finite period of time. The physiological changes occurring in the female reproductive tract rendering the sperm able to fertilize constitute the phenomenon of "sperm capacitation" (Austin, 1952;Chang, 1951). Using the mouse as an experimental model, we have demonstrated that capacitation is associated with an increase in the tyrosine (tyr) phosphorylation of a subset of proteins (Visconti et al., 1995a). We have also demonstrated that the increase in protein tyr phosphorylation as well as capacitation are regulated by a cAMP-dependent pathway (Visconti et al., 1995b). The presence of this regulatory pathway has subsequently been demonstrated in sperm from other species including human (Leclerc et al., 1996;Osheroff et al., 1999). Despite these advances in understanding the mechanisms regulating phosphorylation during capacitation, little is known about the identity of the protein targets of this phosphorylation cascade and of the kinases and phosphatases involved in the regulation of sperm function. Recently, we have used a 2 dimensional (2D) polyacrylamide gel electrophoresis (PAGE) approach combined with tandem mass spectrometry (MS/MS) analysis (Ficarro et al., 2003) to identify proteins that undergo tyr phosphorylation during capacitation of human sperm. In the same work, we have analyzed the exact phosphorylated sequence by MS/MS in proteins from a population of capacitated human sperm and identified sequences phosphorylated in tyr as well as in serine (ser) and threonine (thr) (Ficarro et al., 2003). Among the protein targets, valosin-containing protein (VCP), an ATPase from the same family of the SNARE-interacting protein N-ethyl maleimide soluble factor (NSF) was found to be tyr phosphorylated during capacitation. In addition, immunolocalization of VCP showed a change in fluorescent pattern accompanying sperm capacitation. The hypotheses underlying this proposal combine the above set of findings and postulate that proteins specifically phosphorylated during capacitation as well as the kinases responsible for their phosphorylation are required for the regulation of mammalian sperm capacitation. The objective of this proposal is to further elucidate the sequence of reactions that control protein phosphorylation cascades in mammalian sperm. Characterization of the phosphorylated protein substrates and the kinase(s) involved in the regulation of sperm function will provide novel targets for pharmacological control of the fertilization process.